fix: onOptionSelect should check selectedValue is or isn't be null

[buxuku]

If I press Enter after the input does not match any entries, the component returns a null value and renders an empty component on the page. like:

so we should check the selectedValue is null

[vercel]

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🔍 Inspect: https://vercel.com/react-component/tree-select/J4rpXzTkFJbmtguGKzFdM3kYcMJW
✅ Preview: https://tree-select-git-fork-buxuku-fix-selected-4dc662-react-component.vercel.app

[codecov]

Codecov Report

Merging #396 (56bf0d0) into master (8077330) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #396   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           14        14           
  Lines          462       462           
  Branches       123       128    +5     
=========================================
  Hits           462       462           

Impacted Files	Coverage Δ
src/TreeSelect.tsx	100.00% <100.00%> (ø)

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